1. Field of the Invention
This invention relates to a redox battery and, more particularly to a redox type secondary battery (hereinafter called redox battery) using a vanadium solution of reduction state and an oxidizing substance as a redox couple.
2. Description of the Invention
A redox flow type battery has active materials in electrolytic solution form and charging and discharging are carried out by the redox reaction, by circulating the active materials on positive and negative electrodes in a circulating type electrolytic cell and an intermittent circulating type solution having a membrane, and employing a solution permeable porous electrode. Compared to a conventional secondary battery, a redox flow type battery has the following advantages.
1) In order to increase the storage capacity, only the volume of the storing vessel needs to be increased. The amount of active materials, and, the electrolytic cell itself does not need to be changed unless it is desired to increase the output.
2) Since the active materials on the negative and positive electrodes can be completely separated and stored in each tank, the possibility of self-discharging is less than in those batteries where the active material is contacted with the electrode.
3) With the solution permeating type carbon porous electrode used in this battery the charging/discharging reaction of the active material ions (electrode reaction) is carried out only by exchanging the electrons on the electrode surface, and material is not deposited on the electrode, unlike zinc ions, thus the reaction of the battery is simple.
A redox flow type battery considered to be at a practical level of development at the moment, uses chromium (II, III) and iron (II, III) as a redox couple, and shows excellent performance depending on its use. In long term use, however, the battery has certain defects in that the iron and the chromium cannot be kept from mixing with each other through the membrane of the electrolytic cell resulting in both of the active materials becoming mixtures of iron and chromium. They are thus subject to the limitations of solubility and the concentration of each material cannot be maintained at a high level. Also, in the case of a chromium/iron type battery, the output voltage is around 0.9-1 volt per electric cell, the energy density of the cells (obtained by dividing the amount of energy which can be taken out by discharging, with the cell volume) becomes only around 30 watt hour/liter.
In order to mitigate these shortcomings, a redox flow type battery using chromium chlorine type redox couples, etc. has been suggested (Japanese Patent Laid-Open No. 61-24172), however, an electrolytic solution for a battery composed of an optimal redox couple has not yet been found as shown below.
With chromium, chlorine type redox couples, high concentrations of chloride ion are required since the chlorine is used as the active material. Also, a high concentration of hydrochloric acid is used in order to reduce the electric resistance of an ion exchange membrane used as the membrane, since a cation exchange membrane is chosen for migrating hydrogen ions. However, as the redox potentials of divalent and trivalent chromium ions are close to that of the hydrogen generating potential, the higher the acid concentration becomes, the more the side reaction of hydrogen gas generation is increased, resulting in a lowering of efficiency. Due to the coexistence of high concentration chloride ions, the solubility of chromium ions is decreased as well.
Use of halide acid solutions of iron, copper, tin, nickel and halogen, etc. as an active material to improve the electrode reaction at the positive and negative electrodes, has been suggested (Japanese Patent Laid-Open No. 60-207258), however, none of the combinations are quite satisfactory due to their low electromotive force per electric cell, or complex cell reaction of depositing a metal on the electrode.
Furthermore, an all-vanadium redox flow type battery (J. Electrochem. Soc., 133 1057 (1986), U.S. Pat. No. 4,786,567, using a redox ion couple of vanadium (IV, V) and vanadium (III, II) dissolved in sulforic acid solution as positive and negative electrolytes (J. Power Sources, 15 179, and 16 85 (1985)) has been proposed. However, since it uses expensive vanadyl sulfate, a means to provide inexpensive vanadium is required for the practical use of the technique.
In this context, a redox battery using vanadium ions (III, II) for the negative electrode and halogen or iron ions for the positive electrode has been invented (Japanese Patent Application No. 2-121764). Although the battery had an electromotive force which was comparable to that of an all-vanadium system, since halogen was used as the active material on the positive electrode, it has a problem of requiring a system for storing the active material.
According to U.S. Pat. No. 4,786,567, the electrolytic solution of vanadium is produced from vanadyl sulfate (VOSO.sub.4), by dissolving it in sulfuric acid of 0.1-5 mol/liter (M) and the resulting solution having a vanadium concentration of 0.01-2.5M is used as the battery electrolyte. Since vanadyl sulfate is very expensive as already mentioned it is not suitable for a battery electrolyte to be used in large amounts.
The authors have already proposed a technique to produce a relatively inexpensive vanadium type electrolyte from a vanadium resource in petroleum combustion soot (U.S. Ser. No. 772,794), which is aimed at finding an inexpensive vanadium resource and utilizing a by-product from a Flue gas treatment process. It is therefore an object of the present invention to provide a redox battery having a small size, an improved energy density, and a high output, which can be used for an electric vehicle or as a portable battery, based on the above mentioned finding.